[DOC] Fix tox docs-linkcheck profile issues

[DOC] Fix tox docs-linkcheck profile issues

- ignore URLs logically unreachable such as about:config
- and create a code block for the git clone URL that is not reachable
- ignore URLs with anchors (especially the ones ill-generated
  from external objects, mainly onap-integration)
- enforce tox docs-lincheck profile

Issue-ID: DOC-692
Signed-off-by: Guillaume Lambert <guillaume.lambert@orange.com>
Change-Id: I619d8d5697a35bf2bdf3ee2ed59ca9f5733e7802
(cherry picked from commit 5b9d80a4e9175cfd6bd7f2be33c540053f726005)